Conventionally, in a rolling step of seamless steel tubes and pipes, various mills (continuance mill, sizing mill etc.) have been used, where rolling rolls in these rolling mills are always compressed onto high temperature work material, thus requiring the occasional exchange of rolling rolls since the wear thereof is developed relatively fast and/or the defects on the surface of rolling roll happen to be generated. Also, rolling rolls are exchanged according to the size of work material.
When rolling rolls are exchanged in the above cases, it is essential that, after exchanging rolling rolls, each center of caliber profile formed by each rolling roll mounted onto the stand housing of rolling mill shall be aligned on the same line.
Conventionally, it is common that, in exchanging rolling rolls, rolling rolls are mounted onto a prepared stand housing at a roll shop and then are just polished in that condition, so that a gap between any two adjacent rolling rolls can be adjusted so as to be equal in dimension. Namely, it is a common practice that the stand housing with mounted rolling rolls that are polished is set to the rolling mill and any alignment of all stands is not carried out.
As afore-mentioned, since an alignment of a plurality of stands is not performed, a rolling operation is occasionally carried out while a misalignment remains. The misalignment thus left causes not only a poor dimensional accuracy such as wall thickness, outside diameter, and shape but also defects to be attributable to rolling rolls.
To cope with the above problem, various measuring misalignment methods and alignment methods are proposed and put into practice up to date.
In the first place, as a general method, a method for measuring the position of rolling roll in horizontal direction by comparing the position of the piano wire with a plumb bob, that is hung down from the piano wire tautened along a standard pass-line, to the position shown in a design drawing is commonly known in the case that the operation is suspended for a long time in order to get maintenance, repair or the like done.
Meanwhile, the position in vertical direction is measured by comparing the acquired data by means of the optical leveling instrument to the position in the above drawing, and an adequate adjustment of alignment is made according to the extent of misalignment.
As another alignment method, there is proposed a method for adjusting each pair of rolls so as to make the centerpiece of each jig as stated below coincide with the laser beam center, wherein the laser emitting means is disposed in adjacent to an entrance side of a first stand, and wherein a beam detection device to receive the emitted beam from the above laser emitting means is disposed in adjacent to an exit side of rearmost stand, and wherein a releasable jig with the centerpiece being coincided with the center of the space outlined with an approximate circular shape, that is formed by each pair of calibers (rolls), is provided, and wherein the laser beam is emitted from the above laser emitting means so as to be perpendicular to the side face of the first stand (for example, refer to Japanese Patent Application Publication No. 57-121810).
Also, there is proposed an alignment measuring apparatus comprising a barrel-type jig roll having a standard target in the center, being fitted into the space confined by rolling rolls in each stand of a continuance mill, and an optical reading device capable of detecting the center position of said standard target (for example, refer to Japanese Utility Model Publication No. 03-68901).
Further, there is proposed an alignment apparatus for rolling rolls comprising a light source capable of emitting a parallel beam from the entrance side toward the exit in terms of work material flow in a continuance mill, an optical receiver at the exit side in terms of that being capable of receiving the emitted parallel beam, and a calculation and display device capable of determining and displaying the alignment position by means of the relative position of said rolling rolls being calculated based on the received beam data (for example, refer to Japanese Utility Model Publication No. 04-33401).
Besides, there is disclosed an apparatus for measuring caliber profile off-set comprising a light source and a video camera being disposed both in front of and behind the caliber profile formed by a pair of rolls in a single stand mill, wherein the caliber profile off-set picked up by said video camera is displayed on the display device, thereby enabling the caliber profile off-set to be easily determined (for example, refer to Japanese Patent Application Publication No. 59-19030).
However, in the method for tautening the piano wire as above, there remains an issue that it is merely possible to indirectly identify where the rolling rolls are located with respect to the pass-line and the spatial relationship to the contact position between a rolling roll and work material cannot be directly examined. In this regard, when a wall thickness eccentricity attributable to the misalignment due to the off-set of rolling rolls in a continuance mill is caused, it is not possible to measure the required amount of adjustment, and it has only to be indirectly calculated. Moreover, this kind of adjusting method cannot be applied frequently since it is time-consuming, and the alignment accuracy is within about ±1 mm.
Also, either the prior art disclosed in Japanese Patent Application Publication No. 57-121810 or in Japanese Utility Model Publication-No. 03-68901 relates to a method for measuring the alignment of rolling rolls by the relative positional relationship between the center of the jig, being inserted and fitted into the space confined by rolling rolls, and the emitted laser beam. But the caliber profile formed by three rolling rolls has a complex configuration, and in the case that only one rolling roll is off-set, it is structurally difficult for said jig to be properly inserted and fitted so that the jig center is coincided with the alignment center, thereby it is extremely difficult to assure the alignment accuracy.
Further, the apparatus disclosed in Japanese Utility Model Publication No. 04-33401 is used for measuring an alignment center by getting the profile projection of the groove bottom of rolling roll and there is an issue that an alignment center cannot be measured when the rolling mill is tilted with respect to the optical axis, because the apparatus means that only the spatial relationship of the most convex portion of the caliber profile of rolling roll is determined.
Other apparatus disclosed in Japanese Patent Application Publication No. 59-19030 is configured that a light source is disposed outside the stand, and in the case that the plural stands are provided in such a continuance mill there is an issue that the alignment center of rolling rolls to be measured is not distinguished from that of other irrelevant rolling rolls, because an image of each perimeter profile of a plurality of rolling rolls lies one upon another.
In order to address above-mentioned issues and to perform an alignment measurement accurately in a short time, there is proposed an apparatus for measuring an off-set to be put in place at either entrance side or exit side of a continuance mill, comprising an image-taking device being disposed in such a manner that said device is provided as directed toward said continuance mill and an optical axis thereof approximately coincides with the pass-line of said continuance mill, a lighting device that is put in place in each space between stands that constitute a continuance mill and serves to provide light toward rolling rolls to be measured from the opposite side where the image-taking device is disposed, a signal processing device that calculates the off-set amount of relevant rolling roll based on the taken image of relevant rolling roll by the image-taking device (for example, refer to Patent Application Publication No. 2002-35834).
The above apparatus disclosed in the Patent Publication No. 2002-35834 has an advantage that the alignment center of a continuance mill can be measured in a short time and accurately.
However, in the above apparatus disclosed in the Patent Application Publication No. 2002-35834, the image-taking device must be disposed so that the optical axis thereof approximately coincides with the pass-line of the continuance mill, which is time-consuming and affects the measurement accuracy depending on the extent of coincidence of the pass-line with the optical axis.
Further, as the image is taken by a single image-taking device for each caliber profile formed by rolling rolls mounted on the forefront stand through the rearmost stand, a zoom lens is normally applied as the image-taking optical system for the image-taking device. When the lens with common focal distance is applied as the image-taking optical system, the visual field in image-taking for the forefront stand is significantly differed from that for the rearmost stand, which ends up in abating the resolution capacity in the case of the stand far from the image-taking device, thus resulting in the poor measurement accuracy. Meanwhile, it is well known that the change of focal point normally causes the off-set of visual field (cause the off-set of optical axis). This means that, even if the optical axis is adjusted so as to coincide with the pass-line at predetermined focal position of the zoom lens, the optical axis gets off-set from the pass-line at another focal position. Therefore, it becomes extremely difficult to dispose the image-taking device so that the optical axis thereof approximately coincides with the pass-line of a continuance mill at all focal positions.
As afore-mentioned, in the above apparatus disclosed in the Patent Application Publication No. 2002-35834 to solve the problem encountered in the prior art, it is necessary to dispose the image-taking device so that the optical axis thereof approximately coincides with the pass-line of a continuance mill, which is time-consuming and makes it extremely difficult in the case that the zoom lens is applied as the image-taking optical system for image-taking device, and to dispose the image-taking device so that the optical axis thereof approximately coincides with the pass-line of a continuance mill at all focal positions.